The complexities of golf club design are well-known. The choice of specifications for each component of the club (i.e., the club head, shaft, hosel, grip, and subcomponents thereof) directly impacts the performance of the club. Thus, by varying the design specifications, a golf club can be tailored to have desirable performance characteristics.
The design of club heads has long been studied. Among the more prominent considerations in club head design are loft, lie, face angle, horizontal face bulge, vertical face roll, face progression, face size, sole curvature, center of gravity, material selection, and overall head weight. While this basic set of criteria is generally the focus of golf club engineering, several other design aspects must also be addressed. The interior design of the club head may be tailored to achieve particular characteristics, such as by including hosel or shaft attachment means, perimeter weighting on the face or body of the club head, and fillers within hollow club heads.
The designs for golf club heads must also be strong enough to withstand the impact forces that occur during collision between the head and the ball. The loading that occurs during this transient event can cause an acceleration to the golf ball that is four orders of magnitude greater than that of gravity. Thus, the club face and body should be designed to resist permanent deformation or catastrophic failure, by material yield or fracture.
It is not unusual for club heads of prior art hollow metal woods, produced from titanium, to have a uniform face thickness exceeding 0.15 inches. This thickness has been required to ensure structural integrity of the club head during impact.
Players generally seek a wood and golf ball combination that delivers maximum distance and landing accuracy. The distance a ball travels after impact is dictated by the magnitude and direction of the ball's translational velocity and the magnitude and direction of the ball's rotational velocity or spin. Environmental conditions, including atmospheric pressure, humidity, temperature, and wind speed further influence ball flight. However, these environmental effects are beyond the control of the golf equipment manufacturer. Golf ball landing accuracy is driven by a number of factors as well. Some of these can be attributed to club head design. Primarily, of concern here are center of gravity and club face flexibility.
Golf ball distance is a function of the total kinetic energy imparted to the ball during impact with the club head, neglecting environmental effects. During impact, kinetic energy is transferred from the club and stored as elastic strain energy in the club head and the ball. After impact, the stored elastic energy is transformed back into kinetic energy in the form of translational and rotational velocity of the ball as well as the club. Since the collision is not perfectly elastic, a portion of energy is dissipated in club head vibration and viscoelastic relaxation of the ball. Viscoelastic relaxation is a material property of the polymeric materials used in all manufactured golf balls.
The United States Golf Association (USGA), the governing body for the rules of golf in the United States, has specifications for the performance of golf balls. These performance specifications dictate the size and weight of a golf ball that conforms to the USGA. Furthermore, there are USGA rules which limit the golf ball velocity after a prescribed impact to 250 feet per second ±5%. To achieve greater golf ball distance, ball velocity after impact must be maximized while remaining within these guidelines. Viscoelastic relaxation of the ball is a parasitic energy source, which is dependent upon the rate of deformation. To minimize this effect, the rate of deformation must reduced. This may be accomplished by allowing more club face deformation during impact. Since metallic deformation maybe purely elastic, the strain energy stored in the club face is returned to the ball after impact thereby increasing the ball's outbound velocity after impact.
A variety of techniques may be utilized to vary the allowable deformation of the club face. For example, uniform face thinning, thinned faces with ribbed stiffeners and a varied thickness on the face profile are three possibilities. Any design must have sufficient structural integrity to withstand impact without permanent deformation of the club face.
In general, prior art club heads exhibit large variations in the coefficient of restitution (COR) magnitude with impact location on the face of the club. Furthermore, accuracy of those prior art clubs are highly dependent on this impact location.
Thus, there is a need for a golf club with a face that maximizes golf ball distance while maintaining accuracy due to a reduced sensitivity to face impact location. Furthermore, it would be desirable for the improved club design to minimize the dissipation of spurious energy modes of structural vibration of the club to further maximize efficient energy transfer after impact.